This invention relates to high frequency (HF) communication systems.
High frequency radio has traditionally been a medium used to provide economical, long and short distance non-line-of-sight communications. However, there are problems associated with the use of a high frequency communication system. Some of the problems associated with high frequency systems were disclosed and recommendations made in an IEE (Institute of Electrical Engineers) paper by J. V. Harmon published as part of Colloquium Digest 1979/48, on Feb. 27, 1979, in London.
One method of addressing the problems enumerated in the above reference was addressed in U.S. patent application Ser. No. 4,140,973 entitled, "Channel Evaluation Apparatus For Point to Point Communications." It disclosed a point to point high frequency communication system that has a first set of frequency channels for transmission between a first terminal and a second terminal and a second set of frequency channels for transmitting between the second terminal and the first terminal. The channel evaluation apparatus includes a sounding transmitter and receiver in each terminal. These equipments operate in parallel with the angular communication system and provide the facility for real time evaluation of all assigned channels using a sampling technique. Each sounding transmitter periodically transmits sounding signals and time sequence through its respective frequency channels to the sounding receiver at the other terminal. The sounding signals for each channel are evaluated for transmission quality and this information is displayed. The operator at each terminal may manually or automatically determine the desired channel of reception and this information, which is then encoded on the sounding signals transmitted to the other terminal, is used by the operator at the other terminal to set the communication channels.
U.S. Pat. No. 4,155,040 disclosed an RF communications system that assures selection of the optimum frequency for establishing a communications path between two stations by employing scanning transceivers which may be operated on a plurality of channels having programmed frequencies wherein the programmed frequencies in one transceiver may be automatically changed from another transceiver after establishing a communications path over one of the channels and, when calling another transceiver, the calling transceiver automatically reverts to the same channel last used for communications contact with the called transceiver.